pbc format and pdd13_bytecde

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As discussed in #parrotsketch, pdd13 must document the current pbc format, if it differs from the proposed.

The current problens I've seen are:

The segment header fields itype, id and sized, documented in src/packfile.c in the PackFile_new function, are not mentioned in the pdd. Is not clear if they are intended for depercation or just undocumented yet. The size field is used in several segment types.

The use of the size field is inconsistent. The pir debug segment uses it for the size of his first table, but the annotations segment has his own internal field for each of his tables. Is not clear to me if we must change this to a more coherent usage, or just document current practice for 1.0 and think about changes later.

examples/tools/pbc_checker.cpp shows in a way independent of parrot sources and library how the current practice works, it can read read generated in any platform (at least in those I've been able to test).

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First attempt at pdd13 fix

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Added a patch with the first attempt to reflect current practice in pdd13

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Fixed a confusion in the directory format segment

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Second patch, fixing a confusion in the previous with the directory type segment found when updating pbc_checker.

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Done in r37524, with the addition of a note about the unimplemented state of the Encoding field in string constants.